It is well-known that in case a patient is irradiated by means of penetrating radiation such as x-rays it is most important that the radiation dose to which the patient is exposed is accurately controlled so as to avoid excess exposure which is harmful for the patient.
It is thus extremely important that fluctuations of the emission of a radiation source or deviations of the effectively emitted amount of irradiation from the set amount of irradiation are accurately monitored and adjusted when necessary.
In classical radiography wherein a radiation image is recorded onto a radiographic film, a relation has been established which defines for each specific combination of an intensifying screen and a film the optimal radiation dose to be applied in order to achieve that the diagnostically relevant part of an image is reproduced on the radiographic film at an optimal image density.
Different types of radiographic screen-film combinations have been developed that each provide this optimal reproduction at different irradiation doses. Each of these different types of screen-film combinations is particularly suitable for a specific examination or group of examinations.
The radiation dose suitable for application with a specific radiographic screen-film combination is defined by a value S which is a characteristic of a given radiographic combination called the "speed class". For each radiographic screen-film combination the optimal irradiation dose can be deduced from the speed class value by applying the formula given hereinbelow.
When setting the radiation dose at the deduced value, the diagnostically relevant part of an image will be recorded on a selected film at an optical density which is approximately 1.0 above fog and base density. This speed class value S is defined as follows: ##EQU1## wherein K.sub.s represents the exposure dose required to produce on said film an optical density of 1.00 above fog and base density, said value K.sub.s being expressed in Gy.
For example, when using a class 100 screen-film combination for recording a radiation image, the x-ray dose must be adjusted so that the average dose at the detector (so at the radiographic film) is about 10.sup.-5 Gy in the regions of diagnostical interest which corresponds to an exposure dose of 1.14 mR; when using a screen-film combination of speed class 200 the dose can be halved etc.
Thus, when in classical radiography the radiation dose is set to the value deduced from the definition of the speed class of a selected screen-film combination, the image on the film will always have an identical and predictable density pattern.
Hence an experienced radiologist is able to predict how the density pattern of an exposed part on a radiographic film of a given speed class will look like when the radiation dose is set correctly and hence he will also be able to remark anomalies in the outlook of a diagnostically relevant part that are due to incorrect setting of the radiation source judging from the density of the hard copy. In this way he is immediately and almost automatically warned when the effectively applied radiation dose differs from the required dose because of the direct relation that exists between the amount of radiation transmitted by an object, in case a patient, and the density appearing on the radiographic film.
In digital radiography no such direct relation exists as will be explained furtheron.
In digital radiography a technique has been developed wherein a radiation image of an object is obtained by using a screen comprising a photostimulable phosphor such as described in European patent application 503 702 published on Sep. 16, 1992 and in U.S. Ser. No. 07/842,603.
Upon exposure to penetrating radiation such as x-rays emitted by a radiation source, a photostimulable phosphor screen of the kind described in the above patent applications stores energy. The stored energy can be released by scanning the screen with stimulating radiation, e.g. laser light of the appropriate wavelength. The light emitted upon stimulation which is proportional to the original incident radiation absorption in the screen can then be detected and converted into an electric representation by a read-out system such as a photomultiplier. The gain of the read-out system is commonly adjusted to the expected amount of emitted light by changing the photomultiplier cathode voltage. The read out signal is then digitized.
Preferably the analog image signal is compressed by means of a square root amplifier before being fed to an analog to digital convertor. Compression according to a square root characteristic is advantageous in that it provides optimal use of the dynamic range of the A/D convertor. The result of a compression according to square root characteristic has a constant noise level-throughout the compressed signal range. Since the quantisation error is also constant, the ratio of quantisation noise to signal noise is also constant resulting in an optimal distribution of the bits throughout the dynamic range of the A/D convertor.
After A/D conversion one disposes of an N-bit `raw` image signal, that can be subjected to image processing before hard copy recording or display so that for each type of examination the diagnostically relevant information can be reproduced in optimal conditions.
Among the image processing algorithms a so-called autowindowing algorithm can be applied to determine the diagnostically relevant signal range out of the entire read out signal range.
The latitude and position of this relevant range can be determined automatically using significant points (minima, maxima . . . ) of-the image histogram. A method of determining such significant points has been described in our European application EP 546 600 filed on Dec. 9, 1991 and in U.S. Ser. No. 07/978,786.
The data within the selected window are then mapped onto the density range available on a recording material or on a display. One embodiment of such a signal-to-density mapping method is described in our European application EP 549 009 filed on Dec. 9, 1991 and in U.S. Ser. No. 07/978,091.
The conversion of signal values into corresponding density values is no longer limited by the properties of the recording material, a virtually unlimited number of signal-to-density transformations (or "sensitometries") adapted to each specific application can be selected:
With optimal window-level settings and selection of optimal reproduction sensitometry, it is in most circumstances possible to obtain high quality reproductions. The density pattern obtained on the hard copy is no longer directly related to the given radiation dose and hence the correspondence of the effectively applied radiation dose with the set irradiation dose can no longer be deduced by the radiologist from the density appearing on the hard copy.
A specific embodiment of a procedure followed by an operator when exposing a photostimulable phosphor screen and the implications of this procedure on the detected signal range will be described hereinbelow.
The procedure is based on the operator's knowledge of a classical radiographic system in which a radiation image is recorded on a radiographic film of a speed class suitable for a specific type of examination and wherein the applied radiation dose is deduced from the value of the speed class by application of the above formula.
Exposure of a photostimulable phosphor screen instead of a film of a specific speed class is performed by applying the same radiation dose as would be appropriate for a film of said specific speed class.
Following exposure the image stored in the exposed photostimulable phosphor screen is read by scanning said screen with stimulating radiation (such as laser light of the appropriate wavelength) and by detecting the light emitted upon stimulation by means of a detector such as a photomultiplier. The sensitivity of the detector is adjusted so that the expected read out signal range is centred relative to the detectable signal range (dynamic range of the read out system).
For example in a practical embodiment the detector is a photomultiplier with an adjustable photomultiplier voltage. The sensitivity of the photomultiplier can be changed stepwise, each of the steps for example differing by a factor two. Each setting of the photomultiplier is called a `sensitivity class` in analogy with the notion of `speed classes` used in connection with conventional radiographic film.
The photomultiplier voltage is set to such a value that the expected signal range has approximately equal margins with respect to the minimum and maximum value of the signal range of the read out system.
The data acquisition is commonly done on 12 bit over a range of 2.7 decades on the log E (E being exposure) scale. Depending upon the application the actual object latitude is limited to 1.5 decades on the log E scale or even less. Consequentially a safe margin is available for under- or overexposure.
The image processing algorithms provide that even in case of mal exposure to a limited extent still an acceptable reproduction is obtained, as will be explained hereinbelow.
So, thanks to the image processing even in case of under or overexposure within a limited range still an acceptable reproduction is obtained, but consequentially the result in the reproduction is not directly related to the applied exposure dose and the radiologist or operator can no longer judge from the outlook of the reproduction whether the dose to which the patient was exposed was appropriate for the given application. Since the operator is not warned he may even be stimulated to continue applying said dose.
Indeed, suppose a class 200 exposure was intended and actually 4 times an overexposure (corresponding with a class 50 setting) or 4 times an under exposure (corresponding with a class 800 setting) is performed, this would in either of the cases result in a 0.6 shift on the log E scale. Thus, in either of the cases, the data would still fall within the 2.7 on the log E scale. The autowindowing algorithm, being designed to operate on said 2.7 decades on the log E scale, would still select the relevant data range. Then, this range would be converted into density values so as to obtain a good reproduction or display.